Proposal: dotnet/csharplang#1744

This issue is a place to collect test ideas and track their validation. Use the compiler test plan as a tool for brainstorming.

• validate with LangVersion 7.3
• test with tuple type (SO thread) Issue with tuples and unmanaged constraint #32103
• write speclet (contained under Design section of proposal)
• [ ] update compiler test plan (doesn't appear to be anything to add)
• test public API IsUnmanagedType in new scenarios
• Test for ref structs
• Test the IsManagedType API as suggested by Julien
• Test moveability (existing rules should be sufficient)
• Have at least one test spit some IL to look at and have a few tests run with "expected output" to make sure the CLR can handle the code.
• Stackalloc array of items
• (Mutually) Recursive struct definition
  • Also check the IsUnmanagedType API for these
• Fixed-size buffer inside generic struct
• Also check across metadata (metadata struct definition has private object or unmanaged field)
  • Also private T field
• For all places unmanaged type is required, we should have a test that covers that code path and ensures that the required feature diagnostic is produced at language version < 8.0
  • addressOf &
  • fixed with address of field
  • fixed with array of generic structs
  • sizeof
  • stackalloc with size
  • implicit stackalloc with initializer
  • field of type pointer to unmanaged generic struct
  • unmanaged constraint